Interesting. It seems to me that most civilian high-resolution satellites use flat antennas too. If I recall my SAR lessons correctly, it's because antennas that are short in one direction have a higher resolution when doing SAR.

Maybe a Star motor has flown on top of three different launch vehicles, but I can't think of a liquid stage that pulled the trick except Agena.

I was not trying to be exhaustive.

But Agena was a real workhorse--not only upper stage, but provided power, stabilization and other support for a LOT of different payloads. The information released on the Agena signals intelligence satellites is not very exhaustive, but does indicate that they used gravity gradient stabilization. The Agena pitched over and then stayed that way without any active input. However, later on they discovered that the satellites were nodding slightly, affected by the Earth's magnetic field. It is possible that they had been doing this all along, but the payload accuracy was not high enough for it to show up in the telemetry at first. They corrected the problem with magnets.

The wizard war in orbit (part 1)Early American signals intelligence satellitesby Dwayne DayMonday, June 20, 2016

Tales of espionage are filled with lanky men in trenchcoats walking through cold Berlin streets at the height of the Cold War. But the most important intelligenceŚin terms of volume and reliabilityŚwas gathered by reconnaissance satellites far overhead. These satellites were precise, they collected vast amounts of information, and unlike spies, they did not forget, embellish, lie, or go rogue. Photographic reconnaissance satellites like CORONA, GAMBIT, HEXAGON, and KENNEN were in many ways the most prolific spooks. But they were also accompanied by other satellites, signals intelligence, or SIGINT, satellites that listened for the electronic whispers of radars and radios, engaged in a high-tech war of electrons against an enemy that could vanish and emerge at will.

During the Cold War the United States intelligence community gathered signals intelligence from the Soviet Union via a variety of means. These included ground stations, cable-tapping and bugging operations, airborne platforms such as the RC-135 Rivet Joint and RB-47 Stratojet, and signals intelligence satellites. Any history of SIGINT satellite operations during the Cold War is going to be limited in scope because much of the story remains classified, and unlike the reconnaissance photographs, signals intelligence is an arcane and esoteric subject.

In 1998, the National Reconnaissance Office (NRO), which develops and operates intelligence satellites, declassified the first signals intelligence satellite named GRAB, which was launched in April 1960. GRAB was developed by the Naval Research Laboratory (NSL) in Washington, DC, and part of the NRL was later subsumed as a component of the super-secret NRO. In 2007 the NRO declassified the follow-on to GRAB, named POPPY. But both of these declassification actions were limited, leaving out many major details such as the appearance of some of the satellites, the variety and types of signals they collected, and even how long they operated. The NRO released further details in dribs and drabs over the next several years, but GRAB and POPPY operations remained shrouded in a certain amount of mystery and confusion. But while the GRAB and POPPY revelations were significant, the reality was that they represented only a small part of the story. Throughout the 1960s the NRO operated many other SIGINT satellites and platforms, most developed by the US Air Force, and these remained shrouded in secrecy. Until now.

Interesting. It seems to me that most civilian high-resolution satellites use flat antennas too. If I recall my SAR lessons correctly, it's because antennas that are short in one direction have a higher resolution when doing SAR.

You may be conflating two things: Classical reflector (parabolic section) antennas are often short in one direction. On Earth, that can be useful for a search radar: your radar beam is narrow but high, covering all altitudes at once, but giving good resolution in the horizontal direction.

When you use a phased array, all that no longer applies. You control the beam by coordinating the transmission of each element, using all or part of the array as you wish. Square or rectangular is no longer an issue.

You may be conflating two things: Classical reflector (parabolic section) antennas are often short in one direction. On Earth, that can be useful for a search radar: your radar beam is narrow but high, covering all altitudes at once, but giving good resolution in the horizontal direction.

When you use a phased array, all that no longer applies. You control the beam by coordinating the transmission of each element, using all or part of the array as you wish. Square or rectangular is no longer an issue.

That's not what I remember from my college Microwave Antenna design course. But it's been decades. For a phased array all elements are radiating and you steer the beam through constructive and destructive phase interference. It is the phase (Delay) of each element that you control to steer the beam/pulse. As far as square and rectangular go, the dimensions of the array have a large effect on the shape and compactness of the beam. That said, I'm not a microwave engineer, and the elder Bush was president the last time I looked at it...